by
Kaufmann, Jerome; Schwitters, Karen;

Answer

The answer is below.

Work Step by Step

There are two ways to think of this. First of all, there is the mathematical explanation. When the line is vertical, it is the same as dividing by 0, which is undefined. We can also think of this in terms of practical purposes. If you are climbing up a vertical wall, the slope is infinite, for you are only going up and are not going anywhere in the x direction.